1. Field of the Invention
This invention relates generally to gypsum board and its manufacture, and more specifically, relates to gypsum board having at least one face or surface capable of receiving and adhering to polymeric coatings and that is manufactured quickly and efficiently.
2. Background Art
Gypsum board, and its production, has received attention in the building industry, and especially for providing an easily worked building material the consistency of which is available for general construction use. Desirable characteristics for gypsum board also include a smooth working surface, consistent thickness throughout, and the ability to provide finishing enhancements, such as paint or other protective coverings, thereon.
Recent developments in the manufacture of gypsum board have also added to the durability and versatility of the uses to which gypsum boards may be put.
A particularly useful development in the building board field is known as glass reinforced gypsum (GRG) board. GRG board and its manufacture are well known in the construction industry, and it is described in commonly owned U.S. Pat. No. 4,378,405, incorporated herein by reference. Products made according to U.S. Pat. No. 4,378,405 are sold by the common assignee, BPB, Ltd., under the name “Glasroc.” GRG board, of generally conventional construction, is comprised of a gypsum core having a non-woven glass mat immediately below one or both principal surfaces. In the aforementioned U.S. Pat. No. 4,378,405, the mat is introduced into the core by vibrating the core slurry, which either overlays or underlays the mat, to cause the slurry to pass through the mat, so that the surface layer or layers of gypsum are integral with the core. GRG boards are considered stronger than conventional paper boards and exhibit superior fire resistance.
Manufacture of GRG boards compromises the need to provide strength by employing non-woven glass fiber mat or relatively low diameter (for example, 13 μm (0.005 inch)) fibers with the need to ensure efficient exhaustion of air through a mat from the gypsum slurry from which the board is formed. This is a particular problem at the edge margins of the board where the bottom mat is brought up and onto the upper surface of the board to define the edges of the uncut board. Inefficient exhaustion of air in this region can lead to voids in the edge margins of the cut boards, reducing the edge strength of the boards.
The problem of voids in the edge margins has been dealt with by increasing the fiber diameter of the mat, particularly the bottom mat (to, for example, 16 μm (0.0065 inch)), allowing easier exhaustion of air and penetration of gypsum slurry, but which consequently may result in a reduction of board strength.
Additional compromises in optimization between concerns of cost and of effectiveness arise from the amount of penetration of slurry through the glass mat fibers. In order to ensure that slurry penetrates essentially throughout the surface of the glass mat fibers, aforementioned U.S. Pat. No. 4,378,405 teaches the use of vibration, for example, by vibrators, as disclosed therein. The vibrators vibrate the glass mat and slurry composition to ensure that the “slurry penetrates through the fabric” of the glass mat fibers to form a thin continuous film on the outer surface of the glass mat fibers.
It has been found desirable to form a thin film of slurry on the outer face surface of the glass mat, to avoid exposed fibers of glass, and so to present a smooth working gypsum board surface that can be handled by construction workers without necessitating protective covering of the hands. It has been found that when gypsum boards with exposed glass fibers, such as those taught, for example in U.S. Pat. Nos. 4,647,496; 4,810,659; 5,371,989; 5,148,645; 5,319,900; and 5,704,179, are handled at a construction site by workers, exposed glass fibers penetrate the skin of uncovered hands, and this generally results in worker discomfort. It has been further found that later finishing, e.g., painting, of a smooth gypsum board surface is more desirable because the need for additional pre-finishing steps, such as priming, etc., may be minimized.
Manufacturing facilities for the production of gypsum board, whether or not glass mats are utilized for the structural facings, are capital intensive in the costs of space, equipment and in the down time during which a gypsum board production line is reconfigured. For production of a variety of gypsum board products, for example, standard paper faced gypsum board, glass mat backed board, etc., down time of the production line represents a significant cost in the delay of production of gypsum board and in time wasted by production workers who remain idle.
It has been found advantageous to provide a gypsum board production facility that is easily modified, without long periods of shutting down production, when a production line is being switched from the production of one type of gypsum board to another.
Another consideration in establishing a gypsum board production line arises from the long time required for gypsum slurry in liquid form to be formed, and to set up in a process known as hydration, then to be cut, then processed and dried to remove the water from the set gypsum. To perform the complete process takes a predetermined amount of time, which is an uncompromising restraint on the amount of gypsum board that can be processed on a gypsum board line.
To accommodate these concerns, standard gypsum board lines have been increased in length so that sufficient time elapses as the gypsum travels along the line to permit production, hydration and curing of the gypsum boards, while simultaneously increasing the output of gypsum board being produced on a single board line.
It is important for the board line to run at a sufficient speed, meanwhile maintaining the desired output of gypsum board, while also retaining the efficient operation and consistent quality of the gypsum board produced. Thus, the continuous feed of unset gypsum board is preferably matched with the speed of the conveyor belt as it takes up the gypsum board for the hydration and curing steps occurring down the stream from the gypsum board formation station. Efficient processes for gypsum board must use a production line, therefore that has a length dependent on the rate of desired production, so that the gypsum board becomes fully hydrated and cured at the end of the conveyor belt run.
Additional compromises in optimization between concerns of cost and effectiveness arise from the amount of penetration of slurry through the mineral or glass mat fibers when these are utilized as facing materials. In order to ensure that unset gypsum slurry penetrates essentially throughout the surface of the glass mat fibers, aforementioned U.S. Pat. No. 4,378,405 teaches the use of vibration, for example, by means of vibrators, as disclosed therein. The vibrators vibrate the glass mat and slurry composition to ensure that the “slurry penetrates through the fabric” of the glass mat fibers, to form a thin continuous film on the outer surface of the glass mat fibers.
It has been found desirable to form a thin film of slurry on the outer face surface of the glass mat, to avoid exposed fibers of glass, so as to present a smooth working surface of the gypsum board that can be handled without protective covering of the hands. It has been found that when gypsum boards with exposed glass fibers, such as those taught, for example, in U.S. Pat. Nos. 4,647,496; 4,810,569; 5,371,989; 5,148,645, 5,319,900; and 5,704,179, are handled at a construction site by workers, glass fibers penetrate the skin of uncovered hands and result in discomfort. It has been further found that further finishing, e.g., painting, of a smooth gypsum board surface, is made easier because the need for additional prefinishing steps, such as priming, etc., may be minimized.
Although the smooth surface of gypsum boards provided by the process utilized in aforementioned U.S. Pat. No. 4,378,405 has been found adequate, it is desirable that the operation of the gypsum board line be run quickly and with a more efficient use of available resources. Although the smooth surface of gypsum boards provided by the process utilized in aforementioned U.S. Pat. No. 4,378,405 is adequate to achieve the stated purposes, the process of manufacture, and especially the vibration steps, tend to slow down board production operation and to render the process useful only for specialized applications for which a customer is willing and able to contend with delays in production and in the consequential costs. Moreover, it is not possible to utilize the process of making GRG gypsum boards as taught by U.S. Pat. No. 4,378,405 in a standard gypsum board line because that process requires structural changes to the board production line, which may take time and capital to effectuate.
Another consideration that must be accommodated in terms of timing is the desirability of the gypsum slurry to penetrate through the glass fiber mat so as to produce a clean, smooth surface on the faces of the gypsum board, without unexposed glass fibers extending along the surface. The need to allow sufficient time for the gypsum slurry to penetrate through the mat also restricts the speed of the gypsum board manufacturing line.
It has been found desirable to provide a gypsum board and manufacturing process thereof which can be manufactured at relatively high speed, has high structural integrity and strength by virtue of using a mat of relatively low diameter fibers, and may include in a face coating a polymeric additive material providing a surface ideal for further finishing of the gypsum board. The production process for making gypsum board products according to this invention is capable of quick and efficient change over, for changing of the gypsum board production line, for example, from a board line producing paper faced gypsum board to one producing one or more gypsum boards described herein as embodiments of the gypsum boards according to the present invention.
The present invention can provide an inventive product by utilizing the process according to the present invention and the inventive gypsum board manufacturing facility can provide the capability to quickly change over from a standard plasterboard line, for example, which produces paper backed gypsum boards, to a process utilizing glass mats that become completely covered by a thin film of gypsum, according to the present invention, without requiring breakdown and rebuilding of the production line. The production line, according to this invention, further may be used to produce an embodiment of the present invention which includes a gypsum board having a surface that is relatively smooth and can be utilized or finished without other preparation.